Present Thesis

- PhD thesis of Arthur Beloeuvre

Past thesis

- PhD thesis of Loic LeMeur

"Study of beta decay of exotic nuclei of interest for reactor physics, nuclear structure and
nuclear astrophysics with the Total Absorption Gamma-ray Spectroscopy (TAGS) technique"

Beta decay is involved in many topics in nuclear physics : reactor physics, nuclear astrophysics, nuclear structure… Many experiments have investigated the properties of beta decay of a number of nuclei, the latter being more and more exotic along with technological advances. But in some cases, data suffer from the “Pandemonium” effect. Known since the seventies, this effect is due to the use of high-resolution germanium detectors which have a very low efficiency to detect high energy photons. This may lead to an underestimation of the beta-intensity feeding the high energy levels. To overcome the problem, we use a technique with high intrinsic and geometric efficiencies: the Total Absorption Gamma-ray Spectroscopy (TAGS).

In this work, we will focus on the study of three nuclei, measured at IGISOL in Finland, with the fragmented  detector DTAS. We present the experimental results and the associated beta intensities, and compared them with the existing databases. This comparison shows a non-negligeable amount of beta-intensity that was not detected before for the three studied nuclei. Then we discuss the impacts of these new results on the reactor calculations (antineutrino spectra, decay heat), and analyze the β-feeding of the high energy levels in the daughter nuclei to study Pygmy Dipole Resonances (PDR or low lying resonances). Lastly, our experimental β-strengths are compared with the Gamow-Teller strengths obtained with a QRPA model.

- PhD thesis of Guillaume Pronost

"Etudes pour la mesure de l’angle θ13 avec l’expérience Double-Chooz"

Double Chooz (DC) is an experiment looking at oscillations of electron antineutrinos produced in the Chooz nuclear power plant (France). It aims to measure the θ13 mixing angle, which was, until November 2011, the last non-known mixing angle. The experiment consists in two identical detectors, the FAR detector, running since April 2011 and the NEAR detector, running since December 2014, respectively at  ~ 1 km and ~ 400 m from the reactor cores. DC performs a rate+shape analysis to extract θ13 from the energy spectrum distortion and the deficit due to neutrino disappearance. The NEAR detector measures the neutrino flux and spectrum before oscillations, allowing to normalize the ones after oscillation measured with the FAR detector. In order to provide a precise measurement of θ13, DC needs to control accurately the detector response and the different backgrounds, getting associated systematics as low as possible. This thesis presents the Double Chooz experiment, as well as its analysis and its results, with a specific concern about some systematics reductions. The energy calibration of the detector is detailed; this calibration allows to ensure the mandatory uniformity and stability of the detector response. The background rejection and its measurement are also detailed, with a particular focus on the correlated background. A new rejection method is presented for the correlated background (Gd and H analysis) and for the accidental background (H analysis). Finally, a new alorimetry method is described, taking advantage of the good knowledge of the DC electronics, which improves the photoelectron reconstruction from the photomultipliers signal.

- PhD thesis of Abdoul-Aziz Zakari

"Study of Beta Decay Properties of Fission Product of Interest for Antineutrinos Spectra and Nuclear Reactor Decay Heat"

Fission product beta decay properties play a crucial role in the physics of nuclear reactors, as being the origin of the decay heat, a key parameter for nuclear safety, of beta-delayed neutron emission and of the emission of electronic antineutrinos. However, the decay properties of some fission products remain unknown or incomplete. Indeed in some cases, their data suffer from the “Pandemonium” effect, which is due to the conventional use of high-resolution Germanium detectors, especially when high energy excited levels are populated in the daughter nucleus. This bias may lead to an underestimation of the beta decay branches to higher energy excited levels of the daughter nucleus and constitute a systematic bias in the decay data. An alternative experimental technique is the use of Total Absorption Spectroscopy (TAS), consisting in using a high efficiency detection array. In this PhD, we have measured with the TAS technique the beta properties of 92Rb and 93Rb, two fission products selected for their important contribution to the Pressurized Water Reactor (PWR) antineutrino energy spectrum and in the case of 92Rb, for its contribution to the  235U decay heat. New measurements of the beta decay properties of these nuclei have been performed at the JYFL facility (Jyväskylä, Finland). In this manuscript are presented the performed data analysis, the extracted beta feedings of 92,93Rb nuclei and their impact on the calculation of reactor antineutrino energy spectra as well as on the calculation of the decay heat of the main uranium and plutonium isotopes contributing to the fissions in a PWR.


- PhD thesis of Anthony Onillon

"Estimation of the Chooz cores fission rates and associated errors in the framework of the Double Chooz experiment"

The Double Chooz experiment is designed to search for a non-vanishing mixing angle θ13 characterizing the
ability of neutrinos to oscillate. It consists in two identical detectors located respectively at 400 m and 1050 m of the two pressurized water reactors of the Chooz nuclear plant in the French Ardennes. Indeed, nuclear
reactor are huge electron antineutrino emitters (about 1021  ̄νe/s for a 1GWe reactor). In Double Chooz, antineutrinos are detected by the inverse beta decay process in the liquid scintillator of the detectors:
̄νe+p→e++n. The θ13 parameter can be investigated searching for νe disappearance and  ̄νe energy distortion in the far detector with respect to the near detector. The first phase of the experiment during which only the far detector is taking data has started in April 2011. In absence of far detector whose installation will be completed in 2014, a prediction of the non-oscillated antineutrino flux and spectrum shape expected in the far detector is mandatory to measure θ13. In thismanuscript, we present the simulation work performed to predict the fission rates of both Chooz cores responsible for the reactor antineutrino flux. In this view, a complete core model has been developed with the MCNP Utility for Reactor Evolution (MURE) simulation code. The results of these simulations were used to determine the fission rates and associated systematic errors since the beginning of data taking and led to the first indication for a non-zero θ13 mixing angle in November 2011.

- PhD thesis of Van Minh Bui

"Beta decay of fission products for the non-proliferation and decay heat of nuclear reactors"

Today, nuclear energy represents a non-negligible part of the global energy market, most likely a rolling wheel to grow in the coming decades. Reactors of the future must face the criteria including additional economic but also safety, non-proliferation, optimized fuel management and responsible management of nuclear waste. In the framework of this thesis, studies on non-proliferation of nuclear weapons are discussed in the context of research and development of a new potential tool for monitoring nuclear reactors, the detection of reactor antineutrinos, because the properties of these particles may be of interest for the International Agency of Atomic Energy (IAEA), in charge of the verification of the compliance by States with their safeguards obligations as well as on matters relating to international peace and security. The IAEA encouraged its member states to carry on a feasibility study. A first study of non-proliferation is performed with a simulation, using a proliferating scenario with a CANDU reactor and the associated antineutrinos emission. We derive a prediction of the sensitivity of an antineutrino detector of modest size for the purpose of the diversion of a significant amount of plutonium. A second study was realized as part of the Nucifer project, an antineutrino detector placed nearby the OSIRIS research reactor. The Nucifer antineutrino detector is dedicated to non-proliferation with an optimized efficiency, designed to be a demonstrator for the IAEA. The simulation of the OSIRIS reactor is developed here for calculating the emission of antineutrinos which will be compared with the data measured by the detector and also for characterizing the level of background noises emitted by the reactor detected in Nucifer. In general, the reactor antineutrinos are emitted during radioactive decay of fission products. These radioactive decays are also the cause of the decay heat emitted after the shutdown of a nuclear reactor of which the estimation is an issue of nuclear safety. In this thesis, we present an experimental work which aims to measure the properties of beta decay of fission products important to the non-proliferation and reactor decay heat. First steps using the technique of Total Absorption Gamma-ray Spectroscopy (TAGS) were carried on at the radioactive beam facility of the University of Jyvaskyla. We will present the technique used, the experimental setup and part of the analysis of this experiment.

- PhD thesis of Sandrine Cormon

"Study of the prospective usefulness of the detection of antineutrinos to monitor nuclear power
plants for non proliferation purpose."

The field of applied neutrino physics has shown new developments in the last decade. Indeed, the antineutrinos (νe) emitted by a nuclear power plant depend on the composition of the fuel : thus their detection could be exploited for determining the isotopic composition of the reactor fuel. The International Atomic Energy Agency (IAEA) has expressed its interest in the potentialities of this detection as a new safeguard tool and has created an Ad Hoc working group devoted to this study. Our aim is to determine on the one hand, the current sensitivity reached with the νe detection and on the other hand, the sensitivity required to be useful to the IAEA and then we deduce the required performances required for a cubic meter detector. We will first present the physics on which our feasibility study relies : the neutrinos, the β-decay of the fission products (FP) and their conversion into νe spectra. We will then present our simulation tools : we use a package called MCNP Utility for Reactor Evolution (MURE), initially developed by CNRS/IN2P3 labs to study Generation IV reactors. Thanks to MURE coupled with nuclear data bases, we build the νe spectra by summing the FP contributions. The method is the only one that allows the νe spectra calculations associated to future reactors : we will present the predicted spectra for various innovative fuels. We then calculate the emitted νe associated to various concepts of current and future nuclear reactors in order to determine the sensitivity of the νe probe to various diversion scenarios, taking neutronics into account. The reactor studied are CANadian DeUterium, Pebble Bed Reactor and Fast Breeder Reactor

- PhD thesis of Alberto Remoto

"Etude du bruit de fond induit par les muons cosmiques pour la mesure de l'angle theta_13 avec l'expérience Double Chooz"

- PhD thesis of Mr TALL Yoro 2005-2008

"Mesures de taux de production d’éléments gazeux et volatiles lors de réactions induites par des protons de 1 et 1 ,4 GeV sur des cible épaisses de plomb et plomb-bismuth liquides"